GAMMA-RAY LINES FROM NOVAE

Author

Leising, Mark D.

Date

1987

Degree

Doctor of Philosophy

Abstract

A thermonuclear runaway on the surface of a degenerate white dwarf has been shown to be a plausible scenario for the classical nova phenomenon. This thesis investigates the potential for observing gamma ray line emission from novae, a unique signature of such thermonuclear runaway models. In the high temperature nuclear burning, the low mass nuclei are arranged such that positron unstable nuclei become the dominant isotopes. Upon decay these nuclei produce gamma ray photons either from de-excitation of excited states or from annihilation of the emitted positrons with electrons.
To evaluate the emerging gamma ray fluxes, simple models of the expansion of the nova atmospheres are evolved. These models yield the physical conditions of the outer atmospheres which serve as input to investigations into the fate of the positrons and the transfer of the photons. The latter is accomplished with Monte Carlo simulations of the photon histories. The resulting estimates suggest that nearby fast novae could yield detectable fluxes of annihilation gamma rays from positrons produced by decay of $\sp{13}$N and $\sp{18}$F. It seems less likely that detectable fluxes of de-excitation photons would escape the nova atmosphere at early times, however significant quantities of long-lived nuclei might be produced (i.e., $\sp{22}$Na and $\sp{26}$Al) and yield characteristic gamma rays from the accumulation of the radioactive debris of many novae. Thus novae appear to be a prime target for the Oriented Scintillation Spectrometer Experiment on Gamma Ray Observatory and to offer a potential for the use of the Solar Maximum Mission Gamma Ray Spectrometer as an astronomical observatory.